It is essential for maintaining the performance of an internal combustion engine to change the engine oil that lubricates various parts of the engine from time to time. The interval for changing the engine oil is determined according to the travel distance and period of the use of the vehicle, and the interval for changing engine oil recommended by the manufacturer is set somewhat shorter than is actually necessary to provide a certain safety margin. However, the actual advance of engine oil degradation is so much dependent on the operating condition of the vehicle that changing the engine oil according to the recommended distance and time period of use may result in replacing and discarding the engine oil which is still able to provide an adequate lubricating performance.
Such a conventional practice of changing engine oil means a waste of valuable natural resources, and there has been a need to more accurately determine the interval for changing the engine oil. Commonly assigned U.S. Pat. No. 6,449,538 (Kubo et al.) discloses a method for determining the progress of engine oil degradation according to the engine oil temperature estimated from the cooling water temperature and the condition of cooling water circulation. The entire contents of this patent are hereby incorporated in this application by reference.
As an oil degradation index is known the total base number (TBN) which is a measure of the remaining amount of additives included in the engine oil to keep it clean. As the remaining amount of additives decreases, the capability of the oil to curb the generation of sludge diminishes. Therefore, this number is considered to accurately reflect the practical service life of the engine oil. As a method for estimating the TBN on a real-time basis is known a method based on the measurement of the electric property of the engine oil. See U.S. Pat. No. 7,038,459 (Wakabayashi). The entire contents of this patent are hereby incorporated in this application by reference.
However, the engine oil is stored in an oil pan that communicates with a crankcase into which a large amount of NOx flows depending on the operating condition of the engine. Therefore, the method disclosed in U.S. Pat. No. 6,449,538 that takes into account only the oil temperature may not accurately evaluate the advance of engine oil degradation as it does not account for the influences from the contact with NOx.
The method proposed in U.S. Pat. No. 7,038,459 estimates the acidity or basicity of the engine oil solely from the voltage or static capacitance across a pair of electrodes that are immersed in the engine oil, and the need for a pair of electrodes that are immersed in the engine oil makes this method too expensive and too unreliable to be adopted in vehicles for the general public.